Modern jet aircraft canopies, such as F-22 stealth fighter canopies, are made of polymeric materials. Such materials are preferred because of their light weight, high strength, and ease of shaping. However, most polymeric materials are not infrared-reflective and do not meet the requirements for stealth jet fighters, such as low surface resistance (high electrical conductivity) and the ability to withstand extreme weather conditions. As a result, both organic and inorganic coatings are employed to impart infrared reflection, conductivity, and other necessary stealth characteristics to the canopy.
Canopies for stealth jet fighters need to be electrically conductive so that they can drain or dissipate static electricity. A low surface resistance is desired to prevent a buildup of static charge, because static charge interferes with various electromagnetic field and radar attenuation functions of the aircraft. The canopies should be capable of de-icing and de-fogging, and should provide infrared reflection, radar attenuation, and electromagnetic pulse protection. To meet these requirements, current canopies are coated with several layers of materials to form a “metal conductive stack” or “multilayer stack” i.e., a multilayer coating containing at least one electrically conductive metal or metal oxide layer. However, current canopies do not have good static drain and have poor abrasion resistance.
FIG. 1 illustrates a conventional multilayer stack for a modern aircraft canopy. The multilayer stack 100′ generally includes a substrate base layer 20, a metal conductive layer 40, a tie layer 60, and a top coat 80. The substrate base layer 20 provides adhesion between the metal conductive layer 40 and an aircraft canopy substrate 10 that lies underneath. In addition, the substrate base layer 20 covers imperfections such as scratches or dents that may exist on the surface of the substrate 10. Suitable materials that can be used for the substrate base layer 20 include UV-curable polymers such as acrylates.
The metal conductive layer 40 includes a silver layer 45 and a layer of indium tin oxide (ITO) 50 and helps dissipate static charge that can develop during flight and/or from lightning strikes. In addition, the metal conductive layer 40 provides for electromagnetic interference (EMI) shielding and radar attenuation. Further, the metal conductive layer 40 can be heated for de-fogging, de-misting, defrosting, or deicing, thereby providing a means of clearing any ice or moisture that may accumulate on the top surface of the stack or the inside surface of the canopy. Since the metal conductive layer 40 is prone to oxidation and degradation upon exposure to moisture, the top coat 80 is typically made of aliphatic polyurethane and is sufficiently durable and flexible to withstand the thermal contraction and expansion caused by extreme temperature conditions encountered during flight.
While the prior art multilayer stack provides adequate infrared reflection, EMP protection, and radar attenuation to the canopy, its service life is limited due to delamination and the formation of blue spots on the multilayer stack. In some aircraft, dark blue spots and signs of delamination are observed after only 25 hours of service, and it is not unusual for the entire canopy to be replaced after 80 hours of service because of severe delamination and blue spot formation. The blue spots result from oxidation of the silver layer 45. As moisture penetrates through the top coat 80, it reaches the silver layer 45 and causes it to corrode.
Despite the advances made to date in aircraft canopy coatings, a need still remains for electrically conductive coating stacks having greater durability and functionality.